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We are going to touch only briefly on a parameter expansion in this lecture.

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Let's clear the console first.

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So but we will be covering it extensively in the later just to mention that.

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So it's a feature that is more useful in shell scripts and directly on the command line.

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So many of the capabilities have to do with the system's ability to store small chunks of data and to

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give each chunk a name.

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Many such chunks more properly called R variables are available for your examination.

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So, for example, the variable user contains username to invoke parameter expansion and reveal the

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contents of user.

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You should do this echo dollar sign and user.

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And here, as you can see here, I printed my username so to list of the to see a list of available

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variables.

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You can also try this print any v here and last year we will use this.

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And as you can see here we have this system variables and which is called PG.

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You can.

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And the home directory is this our language log name is iPhone password or.

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No, it's actually PWD.

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Uh, here, uh, this is the working directory that we are in, and here we have, uh, information

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about that.

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So let's actually end this here.

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Oops.

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Here.

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Exit this and here with Ctrl Q or for example, and as you can say, press return lock file is already

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in use.

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So we are use this lock file and here we are going to exit it now.

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With controls.

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So you may have noticed that with other types of expansion and as you can see, we have our current

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working directory is p.w.d..

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So we're going to clear and CD here.

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And again, CD, Let's go home here and let's and let's say we are in the home here.

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So as I said, if you noticed, with other types of expansion, if you mistype a pattern, the expansion

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will not take place and the echo command will simply display the mistyped pattern.

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So with parameter expansion, if you misspell the name of variable, the expansion will still take place

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but will result in an empty string.

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So for example, this.

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So as you remember in previous minutes of our lecture, we printed our username with user.

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But if we write it like that, Echo Sawyer here or yeah.

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Sawyer So actually it's a misspelled type of the user.

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WHEN Okay.

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And as you can see here, we got nothing as an error and it's just an new line.

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So you will also learn about the command substitution in this lecture.

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So common substitution allows us to the output of a command as an expansion.

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So in order to do that, we will echo.

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And here in the inside the braces we will use LHS and here.

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And as you can see, we got this command here.

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So and one of my favorites is this here which and CP and as you can see here, we got this.

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So here we pass the results of which CP as an argument to the command, thereby getting the list of

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a list of the CP program without to know its full path name.

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So we are not limited, just a simple commands entire pipe.

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Pipelines can be used for example, and working with it.

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We also have the cool thing here.

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So now that we have seen how many ways the Shell can perform expansions, it's time to learn how we

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can control it.

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So take for example, this echo.

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This is Oxley and oops, not like this.

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Oh, this is Oxley Academy.

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And as you can see here, this ignored all of these spaces here.

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And you can also do, for example, echo the total.

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Total.

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Price.

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Price.

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Price is, for example, $100.

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And here and as you can see here, the total price is zero.

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So in this first example with this word splitting by the shell, removed extra whitespace from the Echo

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command and the echo commands list of arguments.

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So in the second example, parameter expansion substitute an empty string for the value of $101 because

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it was an undefined variable.

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So the shell provides a mechanism called quoting to selectively suppress unwanted expressions.

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Here we also have the double quotes here, double quotes here.

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So the first type of quoting we will look at is double quotes.

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If a place text inside the double quotes, all the special characters used by the shell lose their special

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meaning and are treated as an ordinary characters.

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So the expressions are the dollar sign and backslash and backtick here.

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Backtick.

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So this means that the word splitting pathname expansion, titled expansion and brace expansions are

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suppressed.

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In this case, however, parameter expansion, arithmetic expansion and common substitution are still

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carried out Using double quotes, we can cope with file names containing embedded spaces.

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Let's say we were the unfortunate victim of a file called this.

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Two words dot txt here.

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If we try to use this on the command line, we are word splitting wood lines called the two words txt.

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And if we try to use this on a command, word splitting will cause this to be treated as a two arguments

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rather than the desired single argument.

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So for example, I want to create a new file, for example.

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Torch torch my file without question mark or something.

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And here and as you can see, we created my file, which now if we write it like that, for example,

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ls l my file my file dot txt.

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This will take as a two arguments, but in order to do access this my file we will use double quotes

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for accessing this here.

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And as you can see here we access to my file here.

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So there.

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And as you can see now, we don't have to keep tying those pesky double quotes.

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So remember, parameter expansions, arithmetic expansions and common substitution still take place

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within the double quotes.

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So for example, look at this.

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So echo and here we will do double quotes.

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So inside this double quotes we will user.

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And dollar sign to.

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The last two.

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Two plus two.

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And after that we will use a dollar sign.

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And after that we will call calendar.

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So this call is actually calendar, which we will show us and calendar is actually not installed.

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So let's actually install calendar.

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So app sudo apt apt, install ankle and I don't think it will take much time because it's I think it's

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is just.

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Here.

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And here.

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This is just a 69kB of file size, and it's already installed here.

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So now let's clear our files.

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Clear and use previous here.

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And as you can see here, we got calendar on it on the screen.

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So we shall take a moment to look at the effect of double quotes on common substitutions.

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So first, let's look a little deeper at words, how words splitting works.

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In earlier lectures, we saw how words splitting appears to remove extra spaces of in our text.

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For example, this is this is Oxley and Here Academy.

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And as you can see here, it just ignored our text.

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So by default, the word splitting looks for the presence of a space tabs and newlines in line characters

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line feed characters and actually and treats them as delimiters between words.

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So this means unquoted space tabs and newlines are not considered to be part of the text, so they serve

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only as separators because they separate the words into different arguments.

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Our example command line contains a comment followed by for district arguments.

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If we add like this, for example, add the quotation marks in the start of the end line and as you

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can see here, it is not ignoring our.

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The new lines or spaces as it did in our previous, like previous example.